Long Term Evolution (LTE), also denoted evolved UTRAN (E-UTRAN), has been defined by the 3rd Generation Partnership Project (3GPP). One of the most important improvement areas in LTE-Advanced is the increase of data rates available for users at the cell edge.
A very promising technique to achieve this goal is the deployment of relays. Relays may broadly be classified into layer 1 relays, layer 2 relays, and layer 3 relays. Layer 1 relays, also known as repeaters do not decode the signal but generally just perform an amplify-and-forward operation. These repeaters only have Layer 1 user plane functionality.
Layer 2 relays demodulate the signal, and typically also perform forward error correction. Depending on the underlying physical layer this demodulation process introduces a non-negligible delay. In case of LTE-Advanced this delay is at least 1 ms and the repeated, delayed signal interferes with new transmissions. On the other hand, the demodulation process removes noise and forwards a “clean” signal. Layer 2 relays have in the user plane in addition to Layer 1 functionality, also Layer 2 functionality.
Layer 3 relays, in the context of LTE, have the same functionality as an eNodeB but the connection of the base station with the network is done via a wireless link using the LTE air interface. Therefore Layer 3 relays are also denoted wireless backhauls. Layer 3 relays may encompass routing functionality.
In LTE Release 8, but also in Wideband Code Division Multiple Access (WCDMA) and other wireless access technologies, Multiple Input Multiple Output (MIMO) is a fundamental concept to increase data rates through spatial multiplexing, also referred to as multilayer transmission. MIMO is further used in order to increase the diversity, i.e. the robustness, of the wireless link. In case of multilayer transmission multiple data streams are simultaneously transmitted over uncorrelated channels to increase the data rates, at least up to a certain degree. Uncorrelated channels are for example achieved for each polarization by separating the multiple transmit and receive antennas sufficiently in space at the transmitter and receiver, respectively. Another possibility is to use polarized antennas. The number of how many layers, i.e. spatial multiplexed streams that can be transmitted simultaneously over a MIMO channel is determined by the channel's rank. To be able to transmit N layers, at least N transmission and reception antennas are required. In order to exploit the sender and receiver antenna arrays optimally as well as to maintain uncorrelated channels even over a multi-hop link via a relay, it is required that the relays maintains the channel rank. In order to do so the relay needs at least as many receive and transmit antennas as the desired end-to-end channel rank. A relay equipped with an insufficient number of antennas collapses the channel and reduces the rank, which is known and denoted as a key-hole effect.
If the input and output antennas of a relay are not sufficiently isolated, then a certain part of the amplified output signals is received by the receive antennas and amplified even further. This effect is denoted self interference. In the worst case the system becomes instable and starts to oscillate. However, even in case the systems remains stable the requirement on the dynamic range of the Analog to Digital Converter (ADC) is increased since the input signal, which probably is rather weak, is interfered by the amplified output signal, which possibly is rather strong. In order to resolve the input signal, a higher resolution of the ADC is generally required.
In order to mitigate the self interference impact, so called self interference cancellation can be exploited.
In the classical approach, the cancellation is done completely in the digital domain. This approach may involve low hardware complexity and few components. Yet, this method dictates a potentially costly ADC with the sufficient resolution to handle the high dynamic range and speed to be used.
The existing method also present weakness when it comes to dynamic range handling, i.e. when the desired input signal may be weaker than the feedback signal to be cancelled. Also, undesired quantization noise of the feedback signal should be avoided.
A relay supporting MIMO transmissions has in the general case N transmit and M receive antennas. In case self interference cancellation is done in digital domain, M ADCs, one for each receive antenna, with high resolution are required.
In case the cancellation is performed in analogue domain, N·M feedback signals need to be cancelled since in total N·M channels exist between M receive and N transmit antennas. A natural approach for a person skilled in the art would hence be to use a Digital to Analogue Converter (DAC) for each signal to cancel. Even though the ADC thus may have a lower resolution, the cost for N·M DAC is substantial when the number of antennas increases.
It is thus a problem to avoid self-interference and at the same time avoiding increasing the costs involved.